Orthopedic implants may be constructed of, or coated with, porous biomaterial to encourage bone growth into the implant. One example of such material is a porous tantalum metal or metal alloy produced using Trabecular Metal™ technology generally available from Zimmer, Inc., of Warsaw, Ind. Trabecular Metal™ is a trademark of Zimmer, Inc. This porous material may be formed of a reticulated vitreous carbon (RVC) bone-like substrate which is infiltrated and coated with a biocompatible material, such as tantalum, in the manner disclosed in U.S. Pat. No. 5,282,861 to Kaplan, the disclosure of which is expressly incorporated herein by reference. The resulting coated material is lightweight, strong, and has an open-cell structure that is similar to the structure of natural cancellous bone, thereby providing a matrix into which cancellous bone may grow to fix the orthopedic implant to the patient's bone.
The starting material for the porous tantalum material is an open-cell polymer foam block or sheet. This polymer foam material is converted into the RVC substrate by first impregnating the polymer foam with a carbonaceous resin and then heating the impregnated foam to a suitable pyrolysis temperature, on the order of 800° C.-2000° C., to convert the polymer foam and any carbonaceous resin into vitreous carbon having individual carbon foam ligaments. The RVC may be shaped into the final form of the orthopedic implant using machining or other shaping techniques. Using CVI, a biocompatible material, such as tantalum, niobium, tungsten, or alloys thereof, may then be coated onto the RVC substrates in a heated reaction chamber. For example, in order to deposit tantalum onto the RVC substrates, solid tantalum metal (Ta) is heated to react with chlorine gas (Cl2) to form tantalum chloride gas, such as tantalum pentachloride (TaCl5), for example. The tantalum chloride gas flows into the reaction chamber and is mixed with hydrogen gas (H2). Upon contact with the heated surface of the substrates, as shown in Equation 1 below, tantalum metal deposits onto the substrates in a thin film over the individual ligaments of the substrates and the hydrogen and chlorine gases react to form hydrogen chloride gas (HCl), which is exhausted from the reaction chamber:TaCl5+5/2H2→Ta+5 HCl  Equation 1
This CVI cycle may be repeated, with the positions of the substrates in the reaction chamber varied, until the substrates are uniformly coated with tantalum. Following each CVI cycle, the hydrogen chloride gas byproduct and any non-converted tantalum chloride gas may react with water and aqueous sodium hydroxide solution to precipitate tantalum oxide, sodium chloride, and water, as is shown in Equation 2:2 HCl(g)+2 TaCl5(g)+H2O(l)+12 NaOH(aq)→Ta2O5(s)+12 NaCl(aq)+8 H2O(l)  Equation 2